I just had the unpleasant experience of doing an rsync --checksum of two
terabytes worth of data. Much to my chagrin, it actually took *longer* than it
would have been to just wipe the filesystem clean and start over, because the
two checksumming passes were done in a serial fashion -- first one machine, then
the other. Each took well over 24 hours to complete.

Please parallelize the filesystem scan phases. There is absolutely no reason
for one machine to sit and wait for the other when it comes to searching its own
filesystem.